The Role of Plastids in the Expression of Nuclear Genes for Thylakoid Proteins Studied with Chimeric [beta]-Glucuronidase Gene Fusions.

We have analyzed plastid and nuclear gene expression in tobacco seedlings using the carotenoid biosynthesis inhibitor nor-flurazon. mRNA levels for three nuclear-encoded chlorophyll-binding proteins of photosystem I and photosystem II (CAB I and II and the CP 24 apoprotein) are no longer detectable in photobleached seedlings, whereas those for other components of the thylakoid membrane (the 33- and 23-kD polypeptides and Rieske Fe/S polypeptide) accumulate to some extent. Transgenic tobacco seedlings with promoter fusions from genes for thylakoid membrane proteins exhibit a similar expression behavior: a CAB-[beta]-glucuronidase (GUS) gene fusion is not expressed in herbicide-treated seedlings, whereas PC-, FNR-, PSAF-, and ATPC-promoter fusions are expressed, although at reduced levels. All identified segments in nuclear promoters analyzed that have been shown to respond to light also respond to photodamage to the plastids. Thus, the regulatory signal pathways either merge prior to gene regulation or interact with closely neighboring cis elements. These results indicate that plastids control nuclear gene expression via different and gene-specific cis-regulatory elements and that CAB gene expression is different from the expression of the other genes tested. Finally, a plastid-directing import sequence from the maize Waxy gene is capable of directing the GUS protein into the photodamaged organelle. Therefore, plastid import seems to be functional in photobleached organelles

Nanobubbles and their precursor layer at the interface of water against a hydrophobic substrate

Neutron reflectivity experiments on the interface of pure D2O against thin films of perdeuterated polystyrene d PS spin coated onto silicon blocks were performed to study the intrinsic structure of the interface of water against hydrophobic substrates. The experiments reveal nonvanishing scattering contrast at the polymer water interface, although the two materials d PS and D2O have closely similar scattering length densities. Organic nondeuterated contaminants or macroscopic air bubbles trapped at the polymer water interface can be ruled out as the origin of this observation. From a systematic study of this system, it is concluded that the source of the nonvanishing contrast is a depletion of water in the boundary layer against the hydrophobic surface. It is conjectured that this depletion layer represents a precursor layer of submicroscopic gas bubbles recently observed by Tyrrell and Attard. The existence of such gas nanobubbles in the present system is confirmed by atomic force microscopy AFM of the surface of d PS coatings in contact with bulk water. The thickness of the precursor gas layer as determined by neutron reflectometry is 2 5 nm, depending on the level of air saturation of the water sample and on the time elapsed after contacting it with the hydrophobic surfac

Characterization of a T-DNA insertion mutant for the protein import receptor at Toc33 from chloroplasts

In Arabidopsis thaliana, the Toc34 receptor component of the chloroplast import machinery is encoded by two independent but highly homologous genes, atToc33 and atToc34. We have isolated a T-DNA insertion mutant of atToc33 which is characterized by a pale phenotype, due to reductions in the levels of photosynthetic pigments, and alterations in protein composition. The latter involve not only chloroplast proteins but also some cytosolic polypeptides, including 14-3-3 proteins which, among other functions, have been proposed to be cytosolic targeting factors for nucleus-encoded chloroplast proteins. Within the chloroplast, many, though not all, proteins of the photosynthetic apparatus, as well as proteins not directly involved in photosynthesis, are found in significantly reduced amounts in the mutant. However, the accumulation of other chloroplast proteins is unaffected. This suggests that the atToc33 receptor is responsible for the import of a specific subset of nucleus-encoded chloroplast proteins. Supporting evidence for this conclusion was obtained by antisense repression of the atToc34 gene in the atToc33 mutant, which results in an exacerbation of the phenotype